Myeloproliferative disorder (MPD) is a heterogeneous group of hematologic diseases which share the common characteristic of myeloid cell overproduction. We have been examining the role of activating mutations of PTPN11, which encodes the protein tyrosine phosphatase, Shp-2, and of c-kit, which encodes the receptor protein tyrosine kinase for stem cell factor (SCF), in juvenile myelomonocytic leukemia and systemic mastocytosis, respectively. GM-CSF signaling via Ras hyperactivation is central to the pathogenesis of JMML; however, we have preliminary studies demonstrating correction of myeloid progenitor GM-CSF hypersensitivity induced by activating PTPN11 mutations by the lipid kinase phosphoinsositol-3-kinase (PI3K) inhibitor, LY294002; therefore, we hypothesize hyperactivation of PI3K activity also contributes to the pathogenesis of JMML. Additionally, in a model of systemic mastocytosis, we have evidence demonstrating that genetic disruption of p85a, a regulatory subunit of class IA PI3K, abrogates mast cell proliferation induced by activating c-kit mutations, leading us to hypothesize that the enhanced proliferation, survival, and migration of mast cells expressing activating c-kit mutations is mediated in part via hyperactivation of PI3K. Therefore, the central hypothesis of this application, formulated on the basis of our preliminary data, is that hyperactivation of class IA PI3K induced by activating PTPN11 and c-kit mutations contributes to the etiology of JMML and systemic mastocytosis, respectively. To examine this hypothesis, we will transduce murine hematopoietic cells lacking expression of the regulatory subunit of PI3K, p85alpha, with activating PTPN11 mutants to conduct in vitro and in vivo hematopoietic progenitor, survival, and proliferation assays as well as biochemical analysis in response to GMCSF stimulation and will utilize a genetic and a biochemical approach involving a direct comparison of the mast cells deficient in p85a or engineered to retrovirally express the activating c-Kit (D814V) mutation to look for modulation of growth, survival and activation of downstream signaling pathways in vitro and MPD in vivo. To define additional potential therapeutic targets in JMML and systemic mastocytosis, we will map the proteome and the phosphoproteome of murine mast cells and stem/progenitor cells expressing the activating mutations of c-Kit (c-Kit D814V) and PTPN11, respectively. Collectively, this combined approach of genetic, biochemical, and proteomic experiments will identify a full range of functions that are controlled by Shp-2 and c-kit via p85 subunits of class IA PI3K and will provide novel targets for molecular therapies in the treatment of JMML and systemic mastocytosis, both of which currently have no good treatment options.